Energy dispersive x-ray spectroscopy (EDS) is a spectroscopic technique predominantly employed in the chemical analysis of specimen. It is based on the investigation of interactions between a high energy beam of particles such as electrons or protons or a beam of x-rays and matter resulting in the release of x-rays from the specimen. The x-ray released is then detected and analyzed by an energy dispersive spectrometer. An EDS setup typically employs a beam source, an X-ray detector; a pulse processor; and an analyzer. A detector converts X-ray energy into voltage signals, which is sent to a pulse processor measuring the signals and passing them onto an analyzer for data display and analysis.
Conventional solid state X-rays detectors usually comprise a semiconductor crystal, such as silicon drifted with lithium (Si(Li)) or high purity germanium (HpGe). Semiconductor detectors measure radiation by means of the number of charge carriers, i.e. free electrons and holes, set free in the detector being arranged between two electrodes. Under the influence of an electric field across the electrodes, the electrons and holes travel to the electrodes, where they result in a pulse, which can be measured. The number of electron-hole pairs is proportional to the energy transmitted by the radiation to the semiconductor.
Silicon drift detectors become more and more popular as x-ray detectors. They provide a cost-effective and high resolving power radiation measurement. Unlike conventional X-ray detectors, such as Si(Li)s, they do not need to be cooled with liquid nitrogen and provide a better noise characteristic and consequently improved energy resolution. Silicon drift detectors consist of a high-resistivity silicon chip where electrons are driven to a small collecting anode. The advantage lies in the extremely low capacitance of this anode allowing very high throughput.
However, as x-ray quanta continue to impinge on the semiconductor crystal, charge accumulates on the electrodes in a ramp-like manner, which eventually saturate. In order to prevent saturation of the electrodes, the accumulated charge must be neutralised, either continuously or by short discharge pulses after which always a new measurement period or ramp can start. With pulsed reset, the detector operation is thus divided in time in (a) a virtually undisturbed measuring period and (b) a possibly noisy reset period.
Pulsed charge reset has been the method of choice with energy dispersive solid state X-ray detection appliances—including Si (Li) detectors, PIN diodes etc.—for many years now. Also Silicon drift detectors with external FET (E-FET SDD) mostly are operated in pulsed charge reset mode. The reason for favouring pulsed charge reset is that means for continuously compensating the detector leakage and signal current without adding additional noise to the critical measurement signal of said detection systems do not exist.
With pulsed reset, the current is allowed to accumulate as a charge on the charge collecting node of the detector until a certain acceptable level is reached, after which the accumulated charge is removed within a relatively short span of time by means of a deliberately increased compensation current.
Different means for introducing the compensation current have been in practical use comprising transistor reset, diode reset, pulsed optical feedback, and pulsed drain feedback.
The inherent loss of useful measurement time and the disturbance of the continuous data stream by the periodic reset periods are in most applications by far outweighed by the improved detector performance, namely energy resolution at high pulse loads, which usually is the primary figure of merit of such detection systems.
Pulsed charge reset approaches classically have been divided into schemas of either resetting charge after each (or a few) hits of a X-ray quantum or resetting only after a relatively long train of events, which—all in all—corresponds to relatively fast or relatively slow triggering of the reset action respectively. The latter procedure is also referred to as ramp-and-neutralising approach.
Practically, only the named ramp-and-neutralising approaches have been of commercial interest, mainly because resetting after each hit of a quantum would waist pulse processing power of the given detector appliance of 50% according to theory and in actual implementations usually much more.
Besides SDD with external FET (E-FET SDD) also SDD with internal FET are know in the art. Practical implementations of pulsed charge reset with commercial SDD have shown limitations of pulse processing power in the range of 100.000 cps, whereas an I-FET SDD detection system inherently can handle more than 1.000.000 cps (in continuous reset mode). This extraordinary pulse processing power is the main advantage of I-FET SDD.
Copying the ramp-and-neutralising approach to silicon drift detectors with internal amplifying FET (I-FET SDD) has been regarded impossible for a long period of time. For instance, Sipilä and Kiuru discuss in US 2005/0285018 A1, which is incorporated by reference herein, that due to stray capacitances inherent to a FET, which change as a function of voltage, applying the ramp-and-neutralise cycle would spread an energy peak obtained as an output of the detector in a hardly predictable manner. Consequently, Sipilä and Kiuru propose controlling means and circuits for resetting charge after (a) each event, (b) a very small number of events or (c) regularly in relatively short periods of time in conjunction with a pulsed drain feedback approach.